1. Field of the Invention
This invention relates to a disc turbine rotor assembly comprised of spaced-apart rotor discs, and provides improvements to the coupling of the disc set to the rotor shaft, and spacing means of the disc members which allow local variation and radial expansion under various local operating temperatures without allowing axial deflection, deformation, or excessive warping of the disc material. Said means maintain desired gaps between planar disc surfaces, and additional spacing and positioning means are provided which establish tangential waves in the disc membranes in order to enhance boundary layer effects. Spoke features of the disc rotor stack are staggered so as to capture working fluid momentum during its axial egress.
2. Description of the Related Art
Turbines comprised of spaced-apart rotor discs were first described by Nikola Tesla in U.S. Pat. No. 1,061,142 and 1,061,206. For this reason, these turbines are sometimes referred to as Tesla Turbines, but are alternatively known as Prandtl layer turbines, boundary layer turbines, cohesion-type turbines, and bladeless turbines.
The turbine rotor consists of a stack of spaced apart discs fixed to a rotatable shaft. The rotor assembly is contained in a housing closely fitted to the perimeter of the discs. The discs have vents near the rotor rotational axis, and the housing includes at least one outlet positioned to receive fluid exiting the rotor assembly. In operation, an energetic fluid at pressure and temperature is introduced at the periphery of the rotor assembly and contained in a housing which closely follows the perimeter of the discs. The fluid passes between the discs and exits the stack assembly through the vents, leaving the housing through its outlets.
In operation, the tangential component of the flow of working fluid creates centrifugal force that must be overcome by additional fluid entering the housing. Therefore, during steady state operation, great back pressure is developed at the inlet of the machine, along with a significant drop in pressure between the inlet and the outlet of the machine. This drop in pressure, with its concomitant drop in temperature and expansion of the working fluid, efficiently extracts much of the available thermodynamic energy of the working fluid.
It is therefore understood that the highest operating temperatures of the working fluid exist at the periphery of the rotating disc assembly, and much lower temperatures exist in the axial regions near the outlets of the turbine. This radial temperature difference along the disc membranes, spokes, and other rotating components presents several material-related challenges, including undesired local and general warping of the disc membranes during extended use and especially after thermal cycles caused by intermittent use or periodically varying working fluid temperatures.
The typical failure mode of a rotor assembly composed of a stack of spaced apart discs is that permanent warping causes non-uniform spaces between the discs, even including closed off or occluded sections where warped material of one disc deflects enough to completely close the gap between it and its neighbor. A correspondingly wide gap area created on the reverse side of the warped disc admits of efficiency losses due to a lack of the close spacing required to maintain effective momentum transfer from working fluid passing through the enlarged section of the passage between the discs.
Prior art devices by Tesla and by Possell (U.S. Pat. No. 4,347,033) include pin members extending axially through several discs. This construction suffices when the device is used as a pump or compressor, but operation as a turbine produces local variations in temperature and differential thermal expansion. Disadvantageously, the pin members accumulate and communicate the forces and stresses of these differential material displacements across the several discs and thereby exacerbate the warping problem.
It is therefore advantageous to provide spacing means which maintain a uniform clearance within the entirety of the gap between opposed surfaces of the spaced-apart turbine discs, but it is also advantageous that these means allow local variation and radial expansion under various local operating temperatures, without allowing axial deflection, deformation, or excessive warping of the disc material.
In comparing the use of spacing means versus the simple robustness of a series of thick plates, it is understood that thinner plates with spacing means provide the same total active plate area at a reduced overall assembly axial thickness, and consequently more power is obtained for a given swept volume of a rotor assembly, thus improving efficiency. Assembly weight and material cost are reduced as well.
A generally known prior art method was initially devised and later taught away by Tesla in British patent 186,082, paragraphs 55-65: xe2x80x9cFurthermore with the object of cheapening the manufacture I dispense altogether with the former spacing studs . . . accomplishing the spacing by means of small bosses or protuberances which are raised in the plates by blows or pressure.xe2x80x9d
Bumps and other features embossed on turbine discs have been employed to limited success as spacing means by individual experimenters and hobbyists to this day. Various inventors, including Conrad, et al, (U.S. Pat. No. 6,183,641) have provided bumps on a first turbine disc surface which do not protrude to closure against any second disc surface. Cafarelli (U.S. Pat. No. 5,470,197) has devised movable means of perturbing the fluid flow between the rotor discs. These inventions, however, do not serve to adequately prevent warping of the disc material or precise and uniform control of the spacing between the discs.
For most turbine applications, maintaining the whole of the disc membranes in flat planar states is desirable for smooth and continuous operation. However, other applications utilize pulsed combustion or pulsed variations of fluid pressures within the turbine. Since the ideal disc spacing for a given fluid at varies with its viscosity, which in turn varies with temperature and pressure, in these applications it is advantageous to present the working fluid with non-planar features within the space between rotor discs so that at any given moment at least some portion of the space between given discs is an optimum space for the fluid condition at any given portion of the pulse.
Another challenge inherent in the design of a disc turbine rotor assembly is the mechanism by which torque imparted to the discs by the working fluid is in turn transmitted to the shaft. Tesla""s invention supplies hubs positioned axially fore and aft of the disc stack and includes threaded fasteners by which the assembly is brought into compression. The shaft, discs, and spacing washers deposed between the discs include at least one keyway, typically accepting of a square key. In the mechanical engineer""s art, keyways are known to be features which concentrate working stresses and substantially reduce the design safety, requiring larger and heavier sections, increasing mass, cost, and design complexity.
In the typical, current, and prior art construction of a disc rotor assembly, the central vents form a radial array, and thereby form a radial array of material connections from a central disc hub to the planar membrane area where the working fluid transfers its momentum to the disc during operation. During this work, direction of fluid motion generally includes tangential and radial components, with very little axial displacement.
However, during egress through the series of vents, the fluid changes its direction from radial to axial motion. This change of motion inheres a change of momentum and therefore a reaction force, but prior art designs release the fluid with without substantially applying this available force to the rotational output of the turbine rotor. Commonly it is observed as a thrust load within the rotor shaft and opposed at the shaft bearings.
To increase the efficiency of the turbine rotor design, it is therefore of benefit to extract useful work from this portion of the working fluid momentum change in addition to the work extracted by the planar surfaces of the discs.
According to the present invention, spaced-apart discs comprising a disc turbine rotor are arranged in an axial stack. Spacing means are required in order to maintain the desired and optimal surface configurations of the planar surfaces of the discs, especially in their planar membrane areas distal to the rotor shaft where operating temperature extremes are greatest.
In the preferred embodiment, warping is prevented by spacing means deposed throughout the disc rotor assembly which maintain a predetermined gap between disc membrane surfaces.
The spacing means of this invention advantageously allow local differential expansion and displacements in radial and tangential directions in response to local variations of material temperature. By allowing these local radial and tangential shifts but without allowing axial deflection, substantially uniform axial clearances are maintained within the entirety of each gap between opposed surfaces of the spaced-apart turbine discs, but no accumulation or communication of forces and stresses of these differential material displacements are imparted from any one disc to any other disc.
Also, because the discs are not rigidly joined, rotor damage which might otherwise be caused by vibration or excessive speed is reduced or eliminated, because local excursions of material in a region of any one disc are not distributed to other discs.
In an additional embodiment preferred for pulsed combustion, the spacing means, plus axial compression of the entire disc array deform the planar surfaces of the discs into a substantially scalloped series of tangential waves. Rather than machining tangential wave features into the discs, discs made from planar sheet material can be deformed into the desired tangential wave shape by radially staggering the spacing means deposed within the disc stack, and then axially clamping the rotor disc stack. Although not within the scope of this invention, common and prior-art clamping designs include sandwiching the rotor disc stack between through-pinned hubs, or between a step feature of the shaft and threaded hub. A variety of snap rings are also available which provide axial take-up, and a pair of these can be installed in receiving grooves in the shaft so as to fix the rotor disc stack in an axially clamped state.
This invention further includes improvements to the design of a disc turbine rotor assembly secured through judicious selection of a rotor shaft cross section and a set of complimentary central apertures in the rotor discs and spacing members. Compared to prior art designs which use keyways or splined shafts, polygons, especially regular polygons, and most especially the hexagon is used to transfer the working fluid forces gathered by the discs into the shaft. Compared to the hexagon, a polygonal hole in a thin sheet having a greater number of sides also has a lower maximum allowable torque beyond which material failure occurs, but countervalingly, for a given cross sectional area, shafts of four or fewer sides are weaker in torsion because the torsional load accumulated in the apices of a shaft section varies according to the square of the distance from the center of the shaft.
Accordingly, several objects of the invention exist:
An object of this invention to provide means permitting radial expansion and contraction of a membrane portion of one individual rotor disc, being part of an axial array of similar discs which together comprise a disc turbine rotor assembly.
Another object is the accurate maintenance of a given and determined axial spacing between a local surface of a membrane portion of a first rotor disc, and a locally opposed surface of a membrane portion of a second rotor disc deposed at a given axial distance apart from said first disc.
A further object of this invention is to provide spacing means which allow for local variation and radial expansion under various local operating temperatures, without allowing axial deflection, deformation, or excessive warping of the disc material.
A yet further object of this invention is to provide a means of maintaining a uniform clearance within the entirety of the gaps between opposed membrane surfaces of a first disc and a second spaced-apart disc included as part of a disc turbine rotor assembly.
A yet further object of this invention is the reduction of total rotor assembly mass, weight, and cost, with its concomitant increase in operating efficiency by means of establishing localized positional control components which locally enforce the desired gap widths between membrane portions of the discs of the rotor assembly, it being understood that such effective spacing means allow a denser array of thinner plates providing larger total active plate area, and consequently secure more power obtained for a given swept volume of rotor assembly.
A yet further object of this invention is also to present a pulsating working fluid with non-planar variations of the space between rotor discs, by providing spacing means throughout the disc rotor assembly which, in assembly, deform the planar surfaces of the discs into a series of substantially scalloped tangential waves. The varying width of the space affords, for fluid at any given state of the pulse, at least one optimial region in which viscous boundary layer effects maximally transfer fluid momentum to the rotor discs.
A yet further object of this invention is the transmission of rotary power from the disc stack to the shaft by means of complimentary cross sections of the rotor shaft, the discs, and the spacing members, so that power transmission capability is enhanced, while mechanical complexity and component cost are reduced.
Another object of this invention is the transmission into rotary output power of useful work from the working fluid and during its direction and momentum changes from a substantially tangential and radial flow direction while at work in between the discs of the rotor stack assembly, to a substantially and axial flow direction while departing the rotor stack assembly.